This invention relates to the preparation of copolyorganosiloxanecarbonates, and more particularly to a method for their preparation which affords a product of high optical clarity; i.e., transparency.
Copolyorganosiloxanecarbonates, also known as siloxane-polycarbonate copolymers and hereinafter frequently designated “siloxane-PC's” for brevity, are of considerable interest since they can combine the advantageous properties of polycarbonates, including ductility, high impact resistance and transparency, with those of polyorganosiloxanes. In particular, block siloxane-PC's are characterized by good flame retardance, mold release properties and low temperature ductility.
Among the known methods for preparation of siloxane-PC's are those in which a carbonyl halide, most often phosgene, undergoes reaction with one or more dihydroxyaromatic compounds, typically 2,2-bis(4-hydroxyphenyl)propane (i.e., bisphenol A), and one or more bis(hydroxyaryl)polyorganosiloxanes. Alternatively, one or both reagents may be the corresponding bischloroformates. A frequently employed bis(hydroxyaryl)polyorganosiloxane may be prepared by the platinum-catalyzed hydrosilylation of eugenol (2-methoxy-4-allylphenol) with a bis(hydride-terminated) polydimethylsiloxane.
While such siloxane-PC's frequently have excellent properties, it is often difficult to prepare them by a method which affords a highly transparent product. The lack of transparency is evidenced by a high haze percentage in molded plaques of the siloxane-PC, and often by similarly high haze in solutions thereof. In general, haze percentages for commercially valuable polymers of this type should be no higher than about 3% for a molded plaque 3.18 mm (0.125 in) thick.
Various methods have been developed for preparation of siloxane-PC's with low haze. For example, U.S. Pat. No. 5,530,083 discloses a multiple-step method in which an oligomeric polycarbonate is prepared by phosgenating a bisphenol in the presence of a phase transfer catalyst to produce chloroformate-terminated oligomers, then condensing a bis(hydroxyaryl)polyorganosiloxane with the chloroformate in the absence of phosgene, and finally removing excess chloroformate, usually by adding an amine as catalyst.
Another method, disclosed in copending, commonly owned application Ser. No. 10/223,037, comprises the steps of, first, contacting a hydroxy-terminated polycarbonate oligomer with a siloxane bischloroformate under interfacial reaction conditions to afford a siloxane-containing polycarbonate intermediate, and, second, contacting the resulting intermediate under interfacial reaction conditions with at least one bisphenol and phosgene. Although this method is simpler than the one disclosed in the aforementioned U.S. Pat. No. 5,530,083 in that it does not require a phase transfer catalyst, it does require two charges of solid bisphenol, one for the preparation of the hydroxy-terminated polycarbonate oligomer and another in the final step, and thus has a longer than desired cycle time.
It would be desirable to produce low-haze siloxane-PC's by a method which uses easily recoverable catalysts, e.g., conventional trialkylamines, rather than phase transfer catalysts alone or in combination with trialkylamines. It would also be desirable to produce them by a method requiring no more than one addition of a solid reactant and adapted to uninterrupted phosgenation when desired, thus avoiding handling complications and extended cycle times.